An aqueous hydrogen peroxide solution is widely used in various fields, for example, for a bleaching agent for paper or pulp and as a component in chemical polishing fluids. In recent years, the aqueous hydrogen peroxide solution has increasingly been used in the electronic industry, for example, as a cleaning agent for silicon wafers and as a cleaning agent in production processes of semiconductors. Accordingly it is required for the aqueous hydrogen peroxide solution that a very high purity be achieved by extremely decreasing all impurities contained in the aqueous hydrogen peroxide solution.
Hydrogen peroxide is produced almost exclusively by the anthraquinone process at present. The anthraquinone process is generally conducted as follows. A derivative of anthraquinone, such as 2-alkylanthraquinone, is converted into anthrahydroquinone by hydrogenation in a water-insoluble solvent in the presence of a hydrogenation catalyst. After the catalyst is removed, the reaction product is oxidized with air to regenerate the original 2-alkylanthraquinone, and at the same time hydrogen peroxide is produced. By extracting the produced hydrogen peroxide from the oxidation product with water, an aqueous solution containing hydrogen peroxide can be obtained. This method is so-called anthraquinone autooxidation method.
The aqueous hydrogen peroxide solution obtained by said method contains metal ion impurities such as Al, Fe, and Cr originated from materials of equipment. Generally, such ion impurities are contained in an amount of tens of mg to a few μg/liter in an aqueous hydrogen peroxide solution having the concentration of 10 to 70% by weight.
When such aqueous hydrogen peroxide solution containing metal ion impurities is used in the field of production processes of semiconductors, the reliability of obtained semiconductors is sometimes considerably lowered. Particularly, recently the required level for reliability of semiconductor becomes much higher. Therefore, an aqueous hydrogen peroxide solution is necessarily purified to a further lower level of concentration of each of metal ion components, for example, to a ppt level.
Conventionally, as a method of purifying and removing metal ion impurities contained in an aqueous hydrogen peroxide solution, there is proposed a method comprising bringing a H+ type strongly acidic cation exchange resin containing a sulfonic acidic group into contact with an aqueous hydrogen peroxide solution. However, merely by contacting the strongly acidic cation exchange resin with the aqueous hydrogen peroxide solution, although metal ion impurities such as Na are removed, it is difficult to remove impurities which are not dissolved completely in the aqueous hydrogen peroxide solution and/or which are originated from metal such as Al, Fe and Cr easily forming a complex with a hydroxide ion. There is a further problem in that a cation exchange resin is deteriorated when coming in contact with the aqueous hydrogen peroxide solution, and thereby, a large amount of sulfate ion is generated from a SO3H group of the cation exchange resin.
In an effort to solve the above problems, there is also known a method that after contact with an aqueous hydrogen peroxide solution and a cation exchange resin, further contact with an OH− type strongly basic anion exchange resin having a quaternary ammonium group is carried out. By said method, the impurities which cannot be removed by a strongly acidic cation exchange resin can be removed.
However, in the use of the OH− type strongly basic anion exchange resin, the aqueous hydrogen peroxide solution is sometimes decomposed when coming in contact with hydroxide ion (OH+) contained in the anion exchange resin, and the decomposition is sometimes further accelerated by the existence of metal ion impurities such as Fe and Cr in the aqueous hydrogen peroxide solution.
In the meantime, as a method removing metal ion impurities while controlling decomposition of an aqueous hydrogen peroxide solution, for example, Japanese Patent Publication No. 35(1960)-16677 discloses that an ion form in an anion exchange resin is converted from a hydroxide ion type into a carbonate ionic type or a bicarbonate ion type, that means lowering the degree of the basicity, and thereby the anion exchange resin can be used. Also, Japanese Patent Laid-open Publication No. 5(1993)-17105 discloses a method that at bringing an aqueous hydrogen peroxide solution into contact with an anion exchange resin, acid is added.
However, in the above cases, undissolved metal ion impurities in an aqueous hydrogen peroxide solution and metal ion impurities which easily form a complex together with a hydroxide ion are not removed completely and remain. Therefore a high purity aqueous hydrogen peroxide can not be obtained. Due to the influence of the metal impurities which easily form a complex together with a hydroxide ion and the remaining metal ion impurities, it is difficult to fully prevent the decomposition of hydrogen peroxide. As a result, it is difficult to purify an aqueous hydrogen peroxide solution safely.
As a method for producing safely and effectively a high purity aqueous hydrogen peroxide solution from which the metal ion impurities, such as Al, Fe and Cr, are removed up to a lower concentration, for example, in the Japanese Patent Laid-open Publication No. 8-73205, there is proposed a method that at purifying an aqueous hydrogen peroxide solution, to the aqueous hydrogen peroxide solution is added acid having an acid dissociation index (pKa) of 5 or less in an amount of 0.05 to 5 milligram equivalent per 1 liter of the hydrogen peroxide solution, followed by bringing the solution into contact with a H+ type strongly acidic cation exchange resin containing a sulfonic acid group or a mixed bed of a H+ type strongly acidic cation exchange resin containing a sulfonic acid group and a strongly basic anion exchange resin.
Further, Japanese Patent Laid-open Publication No. 7-187616 discloses a method using a strongly acidic anion exchange resin having a cross-linkage degree of 5 or less. Japanese Patent Laid-open Publication No. 7-172805 discloses a method using an anion exchange resin converted into carbonate ion type or bicarbonate ion type by ammonium carbonate or ammonium bicarbonate. In Japanese Patent Laid-open Publication No. 7-172805, there is disclosed that the anion exchange resin is used optionally in combination with a cation exchange resin.
Furthermore, Japanese Patent Laid-open Publication No. 8-337405 discloses that metal ion impurities are removed by the use of an ion exchange resin which is primarily contacted with a high purity inorganic acid aqueous solution having a metal ion content of 0.1 ppb by weight or less and then with an ultra-pure water having a metal ion content of 0.1 ppb by weight or less. Japanese Patent Laid-open Publication No. 9-12306 discloses a method that metal ion impurities are removed by the use of an ion exchange resin contacted with an aqueous hydrogen peroxide solution having a metal ion content of 20 ppt or less.
However, by using the above-mentioned methods, metal ion impurities can be removed merely to 1 ppb level. Therefore, aqueous hydrogen peroxide solutions purified by conventionally known methods are difficult to be used for fields such as the electronic industry requiring a high purity quality. Further, some of the above-mentioned conventional purifying methods are not practical to use since the production cost is expensive by using a high purity inorganic acid aqueous solution and an aqueous hydrogen peroxide solution having a metal ion component concentration of 20 ppt or less.
Under these circumstances, the present inventors conducted extensive studies to solve the above-mentioned problems and found that metal ion impurities in an aqueous hydrogen peroxide solution can be removed up to a ppt level (1/1012) by bringing the aqueous hydrogen peroxide solution into contact with, firstly, a H+ type cation exchange resin, secondly, with a carbonate ion (CO32−) type or bicarbonate ion (HCO3−) type anion exchange resin, optionally with a fluoride ion (F−) type anion exchange resin, and thirdly, again with a H+ type cation exchange resin and, further, that the purifying method has the high duplicativity of the metal ion impurities removing level. The present invention has been accomplished on the basis of these discoveries.
There are conventionally known purifying methods comprising contacting an aqueous hydrogen peroxide solution with a cation exchange resin and then with an anion exchange resin and a purifying method using a mixed bed of a cation exchange resin and an anion exchange resin. (For example, see Japanese Patent Laid-open Publication No. 8-73205.) The inventors, however, found, in the methods, metal ion impurities such as Na, K and Al, originating from the final anion exchange resin, elute into an aqueous hydrogen peroxide solution, and thereby exist as metal ion impurity. Furthermore, the inventors found that when the metal ion impurities such as Na, K and Al originated from the anion exchange resin are removed again by the cation exchange resin, metal ion impurities can be removed up to a considerably high purity level and also the duplicativity of the removing level is high.
An object of the present invention is to provide a purifying process of an aqueous hydrogen peroxide solution in which metal ion impurities and the metal ion impurities are removed as completely as possible.